1. Field of the Invention
The present invention relates to a mass spectrometry data analysis method.
2. Description of the Related Art
In mass spectrometry, a polymer that has only one type of repeat unit exhibits only one type of peak interval in a mass spectrum where the horizontal axis is mass-to-charge ratio m/z and the vertical axis is ionic intensity, as illustrated in FIG. 6A. Accordingly, the dispersion state of the degree of polymerization illustrated in FIG. 6B can be read relatively easily from the mass spectrum in FIG. 6A.
Kendrick mass plot is one technique to visualize distribution information of a specimen having a repeat unit (e.g., see Anal. Chem., 2007, 79, p 4074-4082).
In a Kendrick mass plot, a Kendrick mass defect (KMD) is found for each peak appearing in a mass spectrum, and each peak is plotted on coordinates where the horizontal axis is mass, and the vertical axis is the KMD. The KMD refers to the remainder obtained by subtracting the nominal part from the Kendrick mass that is mass recalculated using a reference different from the International Union of Pure and Applied Chemistry (IUPAC) definition where C=12 (generally CH2=14).
That is to say, the analysis method based on the KMD was proposed by Kendrick et al. for elemental composition analysis of saturated hydrocarbon having a repeat unit of methylene (CH2) units. The exact mass of the repeating unit CH2 on the IUPAC mass scale (theoretical value: 14.01565) is defined as Kendrick mass (KM)=14, and the observed exact mass (observed IUPAC mass) of each peak appearing in the mass spectrum is converted into KM using the following Expression.KM=observed IUPAC mass×14.00000/14.01565
The repeating unit structure is not restricted to CH2, and in the case where the specimen is a polymer for example, monomers can be the unit structure. Accordingly, the above expression commonly is written as “KM=observed IUPAC mass of each peak×nominal mass of unit structure/IUPAC mass of unit structure . . . (1)”.
The integer closest to the KM obtained by calculation (the nearest integer of KM) is the nominal KM (NKM), with the KMD being defined as the difference between NKM and KM, i.e., KMD=NKM−KM.
A Kendrick mass plot commonly involves plotting points representing peaks on two-dimensional coordinates where the horizontal axis is the observed mass (m/z), KM, or NKM, and the vertical axis is KMD, with the point being imparted with sizes, color types, darkness, and so forth, in accordance with the intensity information of each peak.
For example, the Kendrick mass plot illustrated in FIG. 7B can be obtained by applying a suitable repeating unit structure (e.g., CH2) to the mass spectrum illustrated in FIG. 7A, obtaining the KMD of each peak by giving the nominal mass and exact mass of the unit structure, and plotting each peak on two-dimensional coordinates where the horizontal axis is the observed mass (m/z) and the vertical axis is KMD, based on the obtained KMD. The Kendrick mass plot illustrated in FIG. 7B indicates the intensity information of each peak by the size of the plotted points.
For example, in a case where there are structures such as expressed by (A)nX in a specimen, where the terminal group (X) is the same and the number of repeats (n) of the repeating unit structure (A) differ, giving the exact mass of the unit structure A for each peak in the obtained mass spectrum to obtain the KMD results in these peaks being plotted on a horizontal straight line on the Kendrick mass plot, since all peaks expressed by (A)nX have the same KMD.
Also, in a case where there are peaks in the same spectrum having the same (A)n repeat unit but a different terminal group (X′), these peaks equally have a different KMD that differs from the case where the terminal group is X, so these peaks are plotted on a horizontal straight line on the Kendrick mass plot, at a different vertical position from the case where the terminal group is X.
On the other hand, in a case where peaks of a different repeat unit (B)n are present in the same mass spectrum, these peaks will be plotted along a straight line on the Kendrick mass plot that is not horizontal but has an inclination.
In the Kendrick mass plot illustrated in FIG. 7B, there were no peaks having the repeating unit structure given to obtain the KMD, so there are no points plotted along a straight horizontal line. Peak groups are divided into and plotted along two straight lines inclined as to the horizontal axis. Looking at the correlation as to the peaks in the mass spectrum in FIG. 7A, the peak group on the upper straight line in FIG. 7B corresponds to the peak group with short repeat intervals in the mass spectrum, and the peak group on the lower straight line corresponds to the peak group with long repeat intervals in the mass spectrum. Accordingly, it can be seen from the Kendrick mass plot in FIG. 7B that there are two types of substances with different repeat units included.
In a case where multiple terminal structures exist, or in a copolymer having two or more types of repeat units, the mass spectrum becomes complicated, and it is difficult to obtain information of the distribution of the degree of polymerization.
In a case where multiple end groups exist, sub-peaks of the terminal structures are observed in a shifted manner, although at the same peak interval, as in the mass spectrum illustrated in FIG. 8 for example, so it is difficult to read the distribution of the degree of polymerization of the repeat units from the mass spectrum.
Also, with copolymers having two or more types of repeat units, peaks having two types or more of intervals are observed as in the mass spectrum illustrated in FIG. 9 for example, although this also depends on the dispersion of degree of polymerization of each component, so a very great number of peaks are observed. Accordingly, it is difficult to read the distribution of degree of polymerization of each repeat unit (repeating A and repeating B at the right in FIG. 9).
Conversely, in a case of employing the analysis method using the Kendrick mass plot, even with a copolymer, ions that have the same the degree of polymerization of one repeat unit out of two repeat units, and a different degree of polymerization of the other repeat unit, are laid out on a straight line, so information of the degree of polymerization of each component is easier to comprehend as compared to a mass spectrum.
That is to say, in a case of a copolymer, even if the mass spectrum is complex such as illustrated in FIG. 10A, employing the analysis method using the Kendrick mass plot plots ions having the same degree of polymerization for each repeat unit on straight lines, as illustrated in FIG. 10B. Accordingly, information of the degree of polymerization of each component can be easily read.
However, in a case where the compositions of two repeat units are similar even if one repeat unit is used as a reference, the deviation of Kendrick mass defect as to the m/z of the two repeat units is in close proximity. Accordingly, the points are close to each other as illustrated in FIG. 11 for example, making it difficult to comprehend the results.